1. Field of the Invention
[The present invention relates to an immersion nozzle, and more particularly to an immersion nozzle for pouring molten steel in a tundish into a mold in continuous casting, or an immersion nozzle made of a refractory material having high corrosion resistance, and capable of suppressing adheres and deposits of matrix metal or nonmetallic inclusion on the nozzle inner wall when used for a long time.
2. Description of the Related Art
In continuous casting, immersion nozzles are used as long nozzle and air seal pipe used between the ladle and tundish, or nozzle for pouring molten steel in the tundish into the mold. These immersion nozzles are required to have such characteristics as spalling resistance, wear resistance and corrosion resistance to molten steel or slag depending on the environments of use.
To meet such requirements, hitherto, alumina-graphite material or alumina-silica-graphite material has been used as the refractory material for immersion nozzle.
In such alumina-silica-graphite refractory material, however, in casting of steel of high oxygen content or type of steel treated with calcium, alumina and silica are eroded by FeO and CaO, and cannot withstand use for a long time.
Accordingly, reducing the content of silica which is relatively low in corrosion resistance, an alumina-graphite refractory material is used, but the resistance of graphite to oxidation and corrosion is low, and sufficient durability is not obtained.
Besides, in the alumina-graphite or alumina-silica-graphite refractory material, the inclusion due to Al oxidation or slag in molten steel adheres and deposits on the inner wall of the immersion nozzle, and the nozzle is likely to be clogged.
If the immersion nozzle is clogged, it is impossible to control the flow rate of molten steel, and operation of continuous casting is difficult, and moreover as the adheres and deposits on the inner wall of immersion nozzle are separated during operation, the quality of product steel is lowered.
As means for preventing such clogging of immersion nozzle, a method of blowing inert gas into the immersion nozzle is proposed. According to this method, adheres and deposits of oxide inclusions such as alumina cluster on the nozzle inner wall can be prevented, but fine bubbles from the blowing gas are taken into the molten steel, and pinholes may be formed, or defects of steel may occur due to entrapping of inclusions. This problem is caused by fine bubbles taken into the molten steel, and it is hence very difficult to solve.
Among other means, from the viewpoint of material of immersion nozzle, as disclosed in Japanese Patent Application Laid-Open No. 56-139260, an immersion nozzle composed of a refractory material containing 5 to 80% of boron nitride has been proposed. More recently, as disclosed in JP-A No. 10-314905, immersion nozzles made of refractory material not containing carbon or lowered in carbon content are proposed in order to prevent adhesion of alumina.
The present inventors noticed the material of the immersion nozzle as possible means for solving the problem of nozzle clogging, and attempted to improve the existing material.
That is, in the immersion nozzle composed of a refractory material containing 5 to 80% of boron nitride proposed in JP-A No. 56-139260, since it is based on the use of graphite, it involved the technical problem of insufficient effect on corrosion resistance and prevention of clogging.
Or in the immersion nozzle made of refractory material free from carbon or lowered in carbon content, although there is an effect of prevention of adhesion sticking of alumina and other oxide inclusion on the nozzle inner wall, there is still a technical problem of insufficient prevention of adhesion of molten steel itself.
Thus, in the conventional technologies about the material proposed as means for preventing nozzle clogging, various problems existed, and the effect for preventing clogging was not sufficient.
The invention is devised to solve the problems of the prior arts, and it is hence an object thereof to present an immersion nozzle having corrosion resistance to steel of high oxygen content or type of steel treated with calcium, and capable of preventing adheres and deposits of molten steel and oxide inclusions on the nozzle inner wall.
In the immersion nozzle of the invention, at least a part of its portion contacting with molten steel is composed of a refractory material containing 50 wt. % or more of a principal component including one or more types of magnesia, alumina, spinel, zirconia, mullite, and silica, and also 0.3 to 15 wt. % of boron nitride, and an organic binder.
This refractory material is low in coefficient of expansion, and small in wettability to molten steel, and moreover since boron nitride excellent in wear resistance, spalling resistance, and lubricity is used, clogging of nozzle is prevented, and an immersion nozzle excellent in durability is presented.
In the immersion nozzle of the invention, further, a thin wall layer of 2 to 15 mm in thickness is formed by this refractory material.
The thin wall layer is preferred to be formed in the specified thickness, at least in the portion contacting with the molten steel, from the viewpoint of nozzle clogging preventive effect and structural peeling of refractory material.